A radial tire for a passenger vehicle typically includes a tire carcass, breaker belt and tire tread. The tire carcass includes radial plies extending from bead to bead and embedded in rubber. The radial plies typically include at least two layers, wherein each layer has a series of reinforcing cords which extend substantially perpendicular to the circumferential direction of the tire.
The belt plies are positioned between the carcass or body plies and the tread. The belt plies similarly include at least two layers, wherein the first layer has a series of reinforcing cords extending at an angle of about 15.degree. to 30.degree. with respect to the circumferential plane of the tire, and the second layer has a series of reinforcing cords extending at an angle of about 105.degree. to 120.degree. with respect to the circumferential direction of the tire, and crossing the first layer. A radial tire is superior in performance--and in particular, comfort, vehicle control, acceleration and braking--to conventional bias tires by virtue of the radial carcass plies and reinforced belt plies.
Another factor which determines tire performance is treadwear. The faster the tire tread wears out during use, the quicker the performance characteristics of the tire are reduced, and the more frequently the tire needs to be replaced. The pattern on the tire tread in large part determines the treadwear of the tire. The tread pattern is segmented into a plurality of raised blocks defined and separated by intersecting circumferential and transverse grooves. The grooves are necessary to provide flexibility and water removal, while the blocks determine the control, acceleration and braking characteristics of the tire. Consequently, the slower the tire tread wears out, the longer the performance characteristics of the tire are maintained and the less frequently the tire needs to be replaced.
The types of irregular tire treadwear include (1) heel-toe wear, (2) cupping, and (3) diagonal wear. Heel-toe wear is the wearing down of the rear portion of a tread block to a greater degree than the front portion of the block. Cupping is the wearing down of several circumferentially adjacent tread blocks in a dish-like manner. Diagonal wear is the excessive wear of a row of tread blocks extending from one shoulder of the tire to the other shoulder at an acute angle to the mid-circumferential plane of the tire. The best measure of the total wear of a tire tread is the weight loss, or the difference between the weight of the tire when it was new and the weight of the tire after it has been in service.
In general, the factors that effect the treadwear of the tire include: (1) the block dimension, (2) the number of ribs, (3) the transverse groove angle, and (4) the shoulder groove subdepth. The dimension of a tread block includes the length of each block in the circumferential direction of the tire. The number of ribs is the number of columns of blocks that extend circumferentially around the length of the tire, with each column being separated by a circumferential groove. Further, the transverse groove angle is the angle at which the transverse grooves, which separate adjacent blocks, extend across the width of the tire. Finally, the shoulder groove subdepth is the depth of a defined portion of the transverse grooves between the blocks.
The block dimension, number of ribs and transverse groove angle determine the amount of tread that contacts the road, and hence the traction and control of the car. The shoulder groove subdepth determines the ability of the grooves to channel water. Moreover, the block dimension, number of ribs, transverse groove angle and shoulder groove subdepth all cooperate in determining the overall performance of the tire--and in particular, in determining the treadwear of the tire.
Prior art tires have attempted to reduce treadwear with various tread designs. For example, a tire manufactured by Yokahama, Model AVS Intermediate, shows a tread pattern wherein at least six ribs of blocks are formed in the tire tread in an asymetrical shape. The transverse grooves on the Yokahama tread pattern extend at an angle of about 45.degree. with respect to the circumferential direction of the tire.